1. Field of the Invention
The present invention relates to a number plate image detecting apparatus for picking up a scene image including a number plate of a vehicle, e.g., an automobile, by using a television camera (to be referred to as a TV camera hereinafter), and detecting an image of a number plate region from the scene image.
2. Description of the Related Art
Recently, a technique of automatically reading number plate information of a vehicle (automobile) entering a toll road such as an expressway has been required to prevent illegal passage by means of exchange of passes, for example, or to automatically collect a toll.
Published Unexamined Japanese Patent Application No. 60-193084 discloses a technique for such a requirement. In this technique, a vehicle which enters a road is detected first by a photosensor, and its front image is picked up by a TV camera. The obtained image data is then subjected to contrast detection by means of spatial filtering in consideration of a contrast between the numerals painted on the number plate and the background. At the same time, binary quantization is performed by using a predetermined threshold value in such a manner that, for example, a numeral portion is set at "1" and a background portion is set at "0". In the obtained binary image, continuous regions of "1" are analyzed to detect an image region having a predetermined size, in which, for example, four numerals are horizontally aligned. With this operation, a number plate region can be detected. In such detection processing, however, since a number plate has a plate frame, when the number plate is inclined with respect to a field of view of the TV camera, the following problem may be posed. In a binary contrast image, continuous regions of numerals and images located close thereto, e.g., continuous regions constituted by horizontal and vertical components of the plate frame may be in contact with each other, interfering with analysis of the continuous regions and causing detection errors.